From WO 88/06544 (U.S. Ser. No. 392,932, incorporated herein by reference) FIG. 7 with associated description it is known, in an anti-lock control system which uses the slip of the wheels as controlled variable, to generate a reference speed signal and an auxiliary reference speed signal for the purpose of slip formation. In the case of the auxiliary reference speed signal, the fastest wheel determines its curve. The gradient of this auxiliary reference speed signal serves only for the determination of the gradient of the reference speed signal during the instability of a wheel, the magnitude of the reference otherwise being determined by the second-fastest wheel.